(1) Recebido para publicação em abril de 2004 e aprovado em janeiro de 2007.

In the densely settled Bragantina region, northern Brazil, smallholder cattle pastures start into compulsory degradation processes indicated, among other factors by the woody encroachment of the resprouting secondary vegetation, nationally called "capoeira". By means of Landsat and QuickBird satellite imagery techniques different stages of treedominated pastures were identified and the distribution of degraded pastures quantified. This was tested in a supervised Landsat classification from 2006 and 2007, taking QuickBird imageries from 2004 and 2005 as additional ground checks. We found that 42% of the rural Bragantinian landscape has already converted into pastures, 28% in advanced stages of above ground pasture degradation. Our reclassification means that the region is still confronted with a serious ecological pasture problem and that conventional land use classifications, climatological models and carbon calculations, based on Landsat imageries only, underestimate pasture distribution and thus lead to inaccurate extrapolations. As the resprouting shrubs of the capoeira are a permanent problem on tropical lowland pastures but, on the other hand, are also the ecological basis of the sustainable slash-and-burn systems of smallholders, we tested some innovative strategies to tolerate various useful woody components of the capoeira on pastures to stabilise them ecologically: first, a buffet trial shows that the ten tested capoeira and some other common domesticated tree species obtain a comparable palatability as well-known woody forage legumes. Thus, Cecropia palmata and Tithonia diversifolia, among others, were as palatable for cattle as the well-known multi-purpose legume Cratylia argentea. As a consequence, these freely accessible tree species should not be slashed but fostered on pastures, like supplementary forage plants. Secondly, it is demonstrated that the multi-purpose forage legumes C. argentea and Flemingia macrophylla perform better under smallholder management than under large scaled farming or even intense management due to the associated prolonged restoration times. In sum, we could demonstrate that woody components (capoeira and/ or woody legumes) are more appropriate for the humid tropics than pure grass-monocultures. Based on these experiences, we modified the model of a grass-capoeira-legume pasture, incorporating pastures just as an interim stage of the slash-andburn system. Forage production could be enhanced in that system by selective slashing of the capoeira (palatable species are tolerated) and by incorporating woody forage legumes. As this biome is known to restore its ecosystem fertility rather in its above-ground biomass than in its poor soils, woody components on pastures are recommended for ecologically sustainable production systems. Furthermore, they might be requisite to mitigate the proclaimed climatic change in the Amazon. Zusammenfassung: Auf neu angelegten Kleinbauernweiden im dicht besiedelten Bragantinagebiet im Norden Brasiliens setzen nach nur wenigen Jahren Degradationspr...

RESUMOEste estudo teve os objetivos avaliar a produção de forragem, o valor nutritivo através de proteína bruta e da composição de macro e micronutrientes na canarana de Paramaribo (Echinochloa polystachya H.B.K) e na canarana erecta lisa (Echinochloa pyramidalis Lam), introduzidas em áreas inundáveis de várzea baixa do rio Guamá, Campus da Universidade Federal Rural da Amazônia, em Belém (PA). As forrageiras haviam sido formadas há três anos e mantidas sob pastejo rotativo, antes e durante o experimento. O delineamento experimental adotado foi o inteiramente ao acaso, em um esquema fatorial 2x4 (duas espécies e quatro períodos), com seis repetições. As amostras de forragem foram cortadas a 10 cm do solo e utilizou-se uma área de 0,25 m 2 como unidade amostral. As épocas de coleta foram: maio (época1), agosto (época2) e novembro/2002 (época3) e fevereiro/2003 (época4). As características analisadas foram: massa fresca, massa seca, teores de nitrogênio, proteína bruta, fósforo, potássio, sódio, cálcio, magnésio, enxofre, ferro, manganês, zinco, cobre e boro. A disponibilidade de forragem diminuiu com o tempo de uso da pastagem. Os teores médios de proteína bruta, N, P, K e Na, Fe, Zn e Cu foram maiores nas épocas mais chuvosas, enquanto que os teores de Ca, Mg, S, Mn e B foram mais elevados nas épocas menos chuvosas. A canarana erecta lisa apresentou menor decréscimo de matéria seca durante os períodos estudados e maior valor nutritivo, no entanto, a canarana de Paramaribo foi a mais produtiva. No entanto, as duas espécies forrageiras apresentaram uma redução expressiva da produção, sugerindo que o tempo de pastejo de 12 dias e/ou o retorno a cada 40 dias constituíram um manejo inadequado. PALAVRAS-CHAVETerra inundável, avaliação, Echinochloa polystachya, Echinochloa pyramidalis. Forage production and nutritive value of forage species under pasture conditions in lowland soils of the Guamá River ABSTRACT This paper had the objective of evaluating forage production, the nutritive value through crude protein, and the macro and micronutrient composition of the canarana de Paramaribo (Echinochloa polystachya H.B.K) and the canarana erecta lisa (Echinochloa pyramidalis Lam) introduced in floodable areas on low and flat lands of the Guamá river, at Universidade Federal Rural da Amazônia Campus, in Belém (PA). The forage plants had been developed for three years and kept under rotative pasture,before and during the experiment. The experimental design adopted was totally randomized, in a factorial 2x4 (two species and two periods) scheme, with six repetitions. The forage samples were cut 10 cm above the soil using a 0,25 m 2 as the sample unit. The collection periods were: May (period1), August (period2) and November/2002 (period3) and February/2003 (period4). The aspects analyzed were: fresh mater, dry mater, nitrogen tenor, crud protein, phosphorus, potassium, sodium, calcium, magnesium, manganese, iron, zinc, copper and borum